Steam power stations (DKW) such as this, in particular steam power stations for generation of electrical power, generally comprise a steam turbine and a fired boiler or, in the form of a gas and steam turbine power installation (GuD), comprise a gas turbine with a downstream waste-heat steam generator and steam turbine.
In the case of fossil-fuelled power stations, the combustion of a fossil fuel leads to the creation of an off-gas containing carbon dioxide. This off-gas generally escapes into the atmosphere. The carbon dioxide which accumulates in the atmosphere impedes the emission of heat from our planet and in the process is leading to an increase in the surface temperature of the planet as a result of the so-called greenhouse effect. Carbon dioxide can be separated from the off-gas in order to achieve a reduction in the carbon-dioxide emission from fossil-fuelled power stations.
Various methods are generally known for separation of carbon dioxide from a gas mixture. One known method is to separate carbon dioxide from an off-gas after a combustion process (post-combustion CO2 capture—Postcap). In this method, the carbon dioxide is separated using a washing agent in an absorption-desorption process.
In one traditional absorption-desorption process, the off-gas is in this case brought into contact with a selective solvent, as the washing agent, in an absorption column. In this case, carbon dioxide is absorbed by a chemical or physical process. The purified off-gas is bled out of the absorption column for further processing or extraction. The solvent loaded with carbon dioxide is passed to a desorption column in order to separate the carbon dioxide and to regenerate the solvent. The separation process in the desorption column can be carried out thermally. During this process, a gas-steam mixture comprising gaseous carbon dioxide and vaporized solvent is forced out of the loaded solvent. The vaporized solvent is then separated from the gaseous carbon dioxide. The carbon dioxide can now be compressed and cooled in a plurality of stages. The carbon dioxide can then be supplied to a storage facility or for reuse, in a liquid or frozen state. The regenerated solvent is once again passed to the absorber column, where it can once again absorb carbon dioxide from the off-gas that contains carbon dioxide.
Thermal power at a temperature level of about 120 to 150° C. is required in order to force the carbon dioxide out of the loaded solvent. This thermal power can be provided by steam which is taken from the steam turbine installation. After passing through the desorption column, the steam is condensed and is passed back again to the steam circuit.
A steam turbine installation generally comprises a high-pressure, a medium-pressure and a low-pressure part. Steam which is introduced into the high-pressure part is expanded in stages via the medium-pressure part and the subsequent low-pressure part. Intermediate superheating is generally carried out between the high-pressure part and the medium-pressure part. The distinction between the medium-pressure part and the low-pressure part is generally distinguished by a steam extraction capability on the overflow line between the medium-pressure part and the low-pressure part.
The extraction of steam from the overflow line for the purpose of CO2 separation is comparable with the outputting of process steam, as is normal practice, for example, for remote heat supply. The amount of extracted steam is in this case dependent on the method of operation of the process steam consumer or the separation apparatus, and may in this case normally vary from 0% to 65%. The amount of steam extracted leads to a reduction in the steam mass flow which is supplied to the downstream turbine stage.
The steam pressure at the extraction point will now fall to the same extent, as a consequence of the steam extraction. The condensation temperature from the heat output also falls with the steam pressure. Since every heat consumer requires a defined temperature level, the steam pressure at the extraction point must not fall below the associated saturated steam pressure. By way of example, a process steam at a pressure level of at least 2.7 bar is required for remote-heat supply with an inlet temperature of 130° C.
In order to overcome this problem, it is known from the prior art for a throttle apparatus to be connected upstream of the low-pressure turbine. It is therefore possible to adjust the pressure in accordance with the temperature required by a corresponding heat consumer. However, this has the disadvantage that the throttling of the remaining steam leads, thermodynamically, to high losses.
Alternatively, the low-pressure turbine can also be adapted for operation with steam extraction upstream of the turbine inlet. For this purpose, modifications are made to the low-pressure turbine either from the start or retrospectively, by means of which the low-pressure turbine is matched to a lower steam mass flow, for the same inlet pressure. By way of example, retrospective modification can be carried out during retrospective installation of a carbon-dioxide separation apparatus in the power station. Known methods for matching the low-pressure turbine to the lower mass flows are to replace one or more rows of blades in order to reduce the choke capability. This method has the disadvantage that, if the process steam consumer or the carbon-dioxide separation apparatus fails or is shut down for desired purposes, at least a portion of the excess steam which occurs in this operating state must be dissipated into the condenser since, otherwise, the pressure and the temperature upstream of the low-pressure turbine would rise impermissibly. Alternatively, the low-pressure turbine and the waste-steam area of the medium-pressure turbine could be designed for higher pressures and temperatures from the start, although this can lead to considerable additional costs.
Steam power stations having process steam consumers as are known from the prior art have the general disadvantage either of losses due to inefficient throttling which is required for operation with a steam output or the loss of excess steam which is created in the operating mode without any steam output, and the steam must be passed to the condenser without being used. These losses lead to an undesirable deterioration in overall efficiency of the steam power station. The efficiency of a steam power station such as this with a process steam output is therefore considerably lower.